This is a great way to teach metric conversions to your child. Study the board first, then I'll explain.
You can see one problem in green, and how it is solved on the chart. We want to know how many kg is 200g? So we go to our chart and start at "g," because it is our base unit. We need to end up at kg, because that is what we are converting to. We jump from one prefix to the next, until we arrive at the unit we need (kg in this case). We jumped 3 to the left. Therefore, we will move our decimal 3 places to the left. As you can see in the drawing, we end up with .200 kg or .2 kg.
In the blue, we want to convert 3m to mm. Again, we start at our base unit, m, and jump until we end up at mm (3 times). We jumped to the right, so our decimal will move 3 places to the right.
Kids will ask...What is hecto? What is deka? What is deci? The truth is, they don't need to know these. Tell them what each prefix stands for, but realize that they will NEVER be asked to convert to these, because we simply don't use them. They are really just place holders. If they leave them out of the chart, they won't jump enough...so while they need to know they exist, they will never use these prefixes. This chart will help them "see" why they are moving the decimal and they will learn pretty quickly how many jumps it is to go all the way from kilo to milli (6) or vice versa (still 6!)
If you have struggled with metric conversions and have some anxiety about teaching them to your child, sit down and make this chart for yourself and then work some problems out. Use this as a TOOL to go along with your child's textbook, which will go more in depth with how the metric system is based on tens, etc. You know your child better than anyone, but I suggest using this chart ONLY with one unit at first, probably meters. Have your child understand how it works first, and THEN say, "Hey, guess what...it works for grams and liters, too!" Trying all three in the beginning is probably too much if you are working with a 3rd or 4th grader. Older kids are less rigid and can assimilate easier with different units.
If you have any questions, please ask!
Here is another great conversion tool!